Development and characterization of an improved Ir--0. 3% W alloy for space radioisotopic heat sources

An Ir--0.3% W alloy has been doped with minor alloying additions to develop an improved cladding material for space radioisotopic heat sources operating at temperatures to 1450/sup 0/C. Tensile tests of a series of doped alloys at slow strain rates indicated that the DOP-4 alloy containing 40 Al, 30 Th, 80 Fe, 10 Ni, and 75 Rh ppm was most resistant to the brittle fracture associated with grain-boundary separation. In addition, the DOP-4 dopants raise the recrystallization temperature, and retard grain growth at high temperatures. The impact properties of DOP-4 and undoped Ir--0.3 W alloys were determined as a function of grain size, test temperature, impact velocity and long-term heat treatment. All the results indicate that the impact properties of the DOP-4 alloy are far superior to those of the undoped alloy. The improvement results from the segregation of beneficial dopant(s) (such as thorium) to grain boundaries and precipitation of second-phase particles. The development of the DOP-4 alloy greatly improves the safety margin of the Multi-Hundred-Watt heat sources to be used in a space probe to Jupiter and Saturn in 1977.